102 lines
3.4 KiB
C
102 lines
3.4 KiB
C
/* Hagelbarger (6,19) code
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*
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* A burst up to 6 encoded bits may be corrupt, to correct them.
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* After corrupt bits, a minimum of 19 bits must be correct to correct
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* another burst of corrupted bits.
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*
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* There is no parity check, so it is required to check all information
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* elements of each message. Messages that contain signals or digits are
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* protected by repeating the digits in the information element.
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*
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* (C) 2017 by Andreas Eversberg <jolly@eversberg.eu>
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* All Rights Reserved
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*
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* This program is free software: you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program. If not, see <http://www.gnu.org/licenses/>.
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*/
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#include "stdint.h"
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/* enable to debug the process of parity check */
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//#define DEBUG_HAGEL
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/* To encode NMT message: (MSB first)
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* Use input with 9 bytes, the last byte must be 0x00.
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* Use output with 18 bytes, ignore the last four (lower) bits of last byte.
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* Use length of 70.
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*/
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void hagelbarger_encode(const uint8_t *input, uint8_t *output, int length)
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{
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uint8_t reg = 0x00, data, check;
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int i;
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for (i = 0; i < length; i++) {
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/* get data from input (MSB first) */
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data = (input[i / 8] >> (7 - (i & 7))) & 1;
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/* push data into shift register (LSB first) */
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reg = (reg << 1) | data;
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/* get data bit from register */
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data = (reg >> 6) & 1;
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/* calc check bit from register */
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check = (reg + (reg >> 3) + 1) & 1;
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/* put check bit and data bit to output (MSB first) */
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output[i / 4] = (output[i / 4] << 2) | (check << 1) | data;
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}
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/* shift last output byte all the way to MSB */
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while ((i % 4))
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output[i++ / 4] <<= 2;
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}
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/* To decode NMT message: (MSB first)
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* Use input with 19 bytes, the unused last 12 (lower) bits must be zero.
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* Use output with 8 bytes.
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* Use length of 64.
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*/
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void hagelbarger_decode(const uint8_t *input, uint8_t *output, int length)
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{
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uint16_t reg_data = 0x00, reg_check = 0xff, data, check, r_parity, s_parity;
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int i, o;
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length += 10;
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for (i = 0, o = 0; i < length; i++) {
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/* get check bit from input (MSB first) */
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check = (input[i / 4] >> (7 - (i & 3) * 2)) & 1;
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/* get data bit from input (MSB first) */
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data = (input[i / 4] >> (6 - (i & 3) * 2)) & 1;
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/* push check bit into shift register (LSB first) */
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reg_check = (reg_check << 1) | check;
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/* push data bit into shift register (LSB first) */
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reg_data = (reg_data << 1) | data;
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/* calculate parity */
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r_parity = (reg_data + (reg_data >> 3) + (reg_check >> 6) + 1) & 1;
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s_parity = ((reg_data >> 3) + (reg_data >> 6) + (reg_check >> 9) + 1) & 1;
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#ifdef DEBUG_HAGEL
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printf("#%d: r=%d s=%d\n", i - 10, r_parity, s_parity);
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#endif
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/* flip message bit, if both parity checks fail */
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/* use 4th bit that will be shifted to 5th bit next loop */
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if (r_parity && s_parity)
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reg_data ^= 0x0008;
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/* put message bit to output (MSB first) */
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if (i >= 10) {
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output[o / 8] = (output[o / 8] << 1) | ((reg_data >> 4) & 1);
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o++;
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}
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}
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/* shift last output byte all the way to MSB */
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while ((o % 8))
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output[o++ / 8] <<= 1;
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}
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